Holography has been widely used extensively for viewing remote objects and measuring characteristics relating thereto. Generally, in such systems, coherent light from a common source is divided into two components by a beam splitter, for example. A first component is then used to illuminate an object, the image of which is to be recorded. A light wave reflected from the object, containing the optical information relating thereto, is directed to a hologram plate member. The second component operates as a known reference and interferes with the reflected signal from the object to create a pattern on the plate member uniquely related to the object. A hologram is the term commonly associated with a diffraction pattern recorded in this manner.
The present invention utilizes techniques employed in holography in order to obtain information relating to the optical figure of a remote object such as a large mirror remotely disposed in space.
Relatively large lightweight mirrors are used in space programs for a number of purposes. The extension of monolithic mirror designs with significantly larger apertures is highly desirable. However, because of the increased structure required for increasing the size of such mirrors, recent development have led to the consideration of thin, actively controlled, deformable mirrors.
A deformable mirror, which may be 2.5 cm or less in thickness, is generally so thin that it must be supported and controlled by a large number of actuators attached to the rear of the mirror. The result is a dynamic optic whose figure must be regularly monitored to ensure proper performance of the system involved.
In order to actively hold the mirror in the proper shape, continuous or frequent measurements of the optical figure of the mirror must be made.
There have been a variety of schemes presented the past for figure sensing from the focal plane; one such method is the full-aperture holographic zone plate. The zone plate approach is attractive since it provides full-aperture figure information which can be easily handled by interferometric figure sensors, and it does not require modification of the mirror surface other than the coating.
The basic idea of the zone plate figure samples is to place in very shallow relief, a weak diffractive optical element (zone plate) on the surface of the primary mirror. This zone plate is an optical element whose center of curvature coincides with the focal point of the primary mirror. When the zone plate is illuminated from the focus of the primary, the zone plate retroreflects the illuminating light. Since distortions in the mirror are translated one-to-one to distortions in the retroreflected light from the hologram, one may indeed sample the figure of the mirror in this fashion.
The hologram could be embossed on the surface of the mirror using photoresist and two coherent point sources. Techniques for exposing the hologram in conventional fashion have been done. Unfortunately, a very large mirror would require very large laser beams and, since the light must be of a narrow spectral range, this means the beams would have very low energy densities. As a result, impractically long exposure times would be necessary. To date, no one has successfully fabricated a zone plate the size of a large primary mirror. Limitations on the laser source make it impractical to obtain large holograms, in the order of meters, from a single exposure.